Not sure how many civilians died in Mariopol, but the reason they died was because Ukranians refused to let them leave in order to use them as meat shields.Why would Ukraine need to kill 50 of its civilians, when Russian forces have already killed 5000 civilians in Mariupol.. ? These extra deaths wouldn't sway any country which has already not been swayed into a hostile position against Russia...
More likely that Russians /DNR/LNR targeted a probable military target in the Railway station, and it hit civilians instead..
And, apart from heightening the optical pressure on Russia for "human rights", there is your reason: stopping civilians from departing.